Recently, there has been desired speed-up of electrophotographic color image forming methods and to realize such speed-up, there are required toners which enable stable color image formation at a high-speed.
Forming color images at a high-speed shortens the time for passing through a nip portion of a fixing device and pressure/heating energy to be provided to a toner is also reduced, often resulting in image defects due to fixing troubles such as offset and rendering it difficult to achieve stable image formation.
Energy saving in electrophotographic image forming apparatuses requires lowering of energy consumed in a fixing device, which consumes a largest amount of electric power in the image forming apparatus. Accordingly, there have been made studies of methods for fixing at a relatively low temperature. To accomplish low temperature fixing, it is necessary to allow a toner to melt at a low fixing temperature, so that there was proposed lowering the melt viscosity of a toner by designation of a low glass transition temperature or a low molecular weight.
However, a toner exhibiting such a low melt viscosity greatly changes in toner viscoelasticity at a temperature neat a fixing temperature, often producing problems that the formed image easily becomes uneven in image glossiness. Further, in such a toner, internal cohesive forces of the melted toner are so low and strength against pulling among toner particles is also weak, producing problems that offsetting easily occurs and it is therefore difficult to obtain a sufficient fixing-allowable temperature range.
To overcome the foregoing problems, there were proposed improvement means noting a storage modulus, as disclosed in, for example, JP-A Nos. 2006-84952 and 2006-133451 (hereinafter, the term JP-A refers to Japanese Patent Application Publication), however, they did not achieve a level meeting the high demands of the market.